1. Field of the Invention
The present invention particularly relates to a two-point modulation phase modulation apparatus modulating a carrier frequency signal using an input digital baseband modulation signal by carrying out two-point modulation using a PLL (Phase Locked Loop), and a polar modulation transmission apparatus, a wireless transmission apparatus, and a wireless communication apparatus using this two-point type phase modulation apparatus.
2. Description of the Related Art
In the related art, phase modulation apparatus employing PLL's where a carrier signal is modulated by a baseband modulation signal so as to form a transmission signal (i.e. a baseband modulation signal is upconverted to a wireless frequency) are widely employed. In this type of phase modulation apparatus, low cost, low power consumption, superior noise characteristics and modulation precision are generally required. In the case of carrying out modulation using PLL's, it is desirable to make the PLL frequency band (PLL frequency band) broader than the width of the frequency band (modulation frequency band) of the modulation signal in order to give good modulation precision.
However, when PLL frequency bandwidth is made wide, this invites deterioration in noise characteristics. Technology referred to as two-point modulation where PLL frequency bandwidth is set to be narrower than modulation frequency bandwidth with modulation being applied at two different locations, modulation within the PLL frequency band and modulation outside of the PLL frequency band, has therefore been proposed (for example, refer to the specification for U.S. Pat. No. 5,952,895).
FIG. 1 shows an example configuration for two-point type phase modulation apparatus of the related art. Two-point modulation phase modulation apparatus 10 has PLL circuit comprised of a voltage controlled oscillator (VCO) 2 with an oscillation frequency changing according to a voltage of a control voltage terminal, frequency divider 3 that frequency-divides an RF phase modulation signal outputted by VCO2, phase comparator 4 that compares an output signal of frequency divider 3 and phase of a reference signal and outputs a signal according to the phase difference, and loop filter 5 that averages and outputs an output signal of phase comparator 4.
In addition to this, two-point modulation phase modulation apparatus 10 has DDS (Direct Digital Synthesizer) 1. DDS1 forms a reference signal based on inputted digital baseband modulation signal S1, and transmits this to phase comparator 4. Phase comparator 4 compares the phases of the reference signal inputted from DDS1 and frequency-dividing signal from frequency divider 3, and transmits a signal corresponding to the phase difference to loop filter 5. As a result, two-point modulation phase modulation apparatus 10 carries out modulation of the first point based on input digital baseband modulation signal S1.
Further, two-point modulation phase modulation apparatus 10 has D/A converter 6 that analog-converts input digital baseband modulation signal S1, anti-alias filter 7 that suppresses alias components contained in output signal S2 of D/A converter 6, and adder 8 that adds output signal S3 of anti-alias filter 7 and an output of PLL circuit loop filter 5 and outputs the added signal to a control voltage terminal of VCO2. As a result, two-point modulation phase modulation apparatus 10 carries out modulation of the second point based on input digital baseband modulation signal S1.
When this kind of two-point modulation technology is used, it is possible to output a wide frequency band RF phase modulation signal that goes beyond the PLL frequency band even if the PLL frequency band is set to be narrower than the modulation frequency band. As a result, it is possible to suppress deterioration of noise characteristics due to PLL.
FIG. 2 is a view showing a frequency characteristic for a baseband region for illustrating operation of the two-point modulation phase modulation apparatus. Here, a transfer function expressing the PLL frequency characteristic is taken to be H(s) (where s=j ω). H(s) has a low-pass characteristic as shown in FIG. 2. A low-pass filter of transfer function H(s) is then applied by the PLL to input digital baseband modulation signal S1 inputted to the PLL circuit via DDS1 and phase comparator 4. On the other hand, a high-pass filter of transfer function 1-H(s) as shown in FIG. 2 is applied to input digital baseband modulation signal S1 inputted to PLL circuit via D/A converter 6, anti-alias filter 7 and adder 8. Namely, when input digital baseband modulation signal S1 is taken to be Φ (s), a baseband component contained in an RF phase modulation signal outputted by voltage controlled oscillator 2 bears no relation to a PLL frequency characteristic, as shown in the following equation.H(s)Φ(s)+{1-H(s) }Φ(s)=Φ(s)  (1)
As described above, when two-point modulation is applied to the PLL, a baseband modulation signal component mainly within the PLL frequency band is transmitted from loop filter 5 to VCO2, while baseband modulation signal component mainly outside of the PLL frequency band is transmitted from anti-alias filter 7 to VCO2. As a result, at VCO2, baseband modulation signal components for within the PLL frequency band and outside of the frequency band are added together, and it is possible to output a wide band RF phase modulation signal that goes beyond the PLL frequency band.
In addition, it is necessary for input digital baseband modulation signal S1 to be the dimension of the frequency. VCO2 acts as an integrator and an RF phase modulation signal outputted by VCO2 is therefore converted to a dimension of phase by VCO2. Here, for example, the dimension of the GSM scheme baseband modulation signal is typically phase. As a result, in reality, with baseband modulation signals such as in GSM schemes where the dimension is phase, after differentiation to give conversion to a frequency dimension, this is inputted as input digital baseband modulation signal S1 of FIG. 1.
Here, a description is given of the operation of anti-alias filter 7 using FIG. 3. The input digital baseband modulation signal S1 shown in FIG. 3A is converted to an analog signal S2 by D/A converter 6. As shown in FIG. 3B, with an analog signal S2 outputted by D/A converter 6, an alias signal that is half the frequency of the sampling frequency (fs) of D/A converter 6 wrapped around a frequency axis at a border is generated. This alias signal is then suppressed by the frequency characteristics of the anti-alias filter 7. As a result, as shown in FIG. 3C, a baseband analog modulation signal S3 without alias component is outputted from anti-alias filter 7. The horizontal axis of FIG. 3 is shown to take the Log of the frequency.
It is necessary to sufficiently suppress the alias signal in order to satisfy the specifications for noise outside of the wireless frequency band. In order to achieve this, it is therefore preferable to take large amount of attenuation of anti-alias filter 7 at the frequency of the alias signal. However, if the frequency bandwidth of anti-alias filter 7 is made too narrow in order to make the amount of suppression of the alias signal (the right side portion of the analog signal S2 of FIG. 3B) large, an original baseband modulation signal component (the left side portion of the analog signal S2 of FIG. 3B) is suppressed and modulation precision deteriorates.
Typically, it is possible to set a sampling frequency fs sufficiently high compared to the baseband modulation frequency bandwidth so as to make the generation frequency of the alias signal high. As a result, it is possible to take large amount of the attenuation of the generation frequency of the alias signal even if the frequency bandwidth of the anti-alias filter 7 is made sufficiently broader than the baseband modulation bandwidth.
However, in wireless communication in recent years, as shown in FIG. 4A, cases where a wide frequency band signal is used as baseband modulation signal S1 are common. In this case, if only the alias signal component (the right side portion of the analog signal S2 in FIG. 4B) is suppressed without suppressing the original baseband modulation signal component (the left side portion of the analog signal in FIG. 4B), as shown in FIG. 4B, an anti-alias filter 7 is required where the frequency bandwidth is broader than the frequency bandwidth of the PLL that rapidly changes the frequency characteristic between the original baseband modulation signal component and the alias signal component. Namely, a high-order anti-alias filter 7 is necessary. However, increasing the order of anti-alias filter 7 means increasing the number of dependent circuits consisting of resistors and capacitors, and the configuration of the anti-alias filter 7 therefore becomes complex.
On the other hand, as described above, a method of increasing the sampling frequency fs of the D/A converter 6 is considered as a method of suppressing only the alias signal component without increasing the order of the anti-alias filter 7 (i.e. without complicating the configuration of anti-alias filter 7). However, it is necessary to increase the clock frequency in order to increasing the sampling frequency fs of D/A converter 6 which raises another problem of the power consumption increasing as a result.